A standard extrusion head for a blow-molding apparatus has an annular and relatively rigid outer housing centered on a normally vertical axis, a sleeve coaxially received in the housing and having an upstream end secured to the housing and a radially elastically deflectable downstream end, and a rigid core coaxially received within the sleeve in the housing and having an outer surface defining with an inner surface of the sleeve an annular passage. Actuators mounted on the housing are braced radially against the downstream sleeve end so as to be able to move and/or deform it radially. Furthermore the core is typically displaceable axially so as to change the average radial width of the passage since the outer surface of the core and the inner surface of the sleeve both flare axially downward at the downstream end.
When blow molding an irregularly shaped object, as for instance the above-mentioned motor-vehicle fuel tank, some parts of the tubular parison or tube exiting the downstream end of the passage are going to be subjected to substantially more stretching in the mold into which the parison is extruded than other parts. For instance if the filler mouth for the tank, which is typically formed directly by the parison, is closer to the back wall of the tank than to the front wall, the parison will be stretched more to make the front wall than to make the back wall.
Since the goal is to have a finished workpiece of uniform wall thickness, it is therefore necessary to make the parison thicker in those regions subjected to a great deal of stretching and thinner where it will not be stretched so much. This can be accomplished by computer-controlled operation of the actuator that controls the axial position of the core and the actuators that radially deform and/or displace the downstream end of the passage-defining sleeve.
In a standard system (Plastverarbeiter 32, 1981, no 3, pages 326-330) there are two double-acting actuators aligned along a diametral main axis and bearing on the sleeve. Both actuators can exert a radial outward tension or a radial inward compression to form the sleeve into an ellipse, or they can both exert a radial force in the same diametral direction to shift the sleeve, while maintaining it circular, to one side or the other. While such an arrangement is fairly effective, those portions of the passage offset by 45.degree. to the main axis are not under control and it is virtually impossible to locally increase or decrease passage width in these regions.
In German 196 03 231 an extrusion head of this type is known which has connected to the core an elastically deformable sleeve that is radially deformably by four crossing spreader elements. The core actuates these spreader elements synchronously so that the passage can only be changed in a symmetrical manner. Once again there are regions left that cannot be controlled as to width.
Finally, in German 195 37 132 the nozzle housing itself is radially shiftable. Two angularly offset adjustment rings are responsible for the radial movement. This device is used to blow-mold tubing elbows where only the limited amount of adjustability provided is sufficient to locally thicken the parison where it forms the outside of the corner of the finished part.